1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming system.
2. Description of Related Art
There are known some conventional sheet processing apparatuses each of which forms a booklet with a saddle stitching section including a saddle unit to sequentially accumulate sheets of paper that are mountain-folded along middle folding lines so as to straddle the saddle unit, a staple inserting unit to insert a staple from above into the folding lines of the sheets accumulated on the saddle unit and a staple receiving unit which is provided to face the staple inserting unit across the sheets and receives and bends the ends of the staples inserted by the staple inserting unit.
In a saddle stitching section described in Japanese Patent Application Laid Open Publication No. 2010-241523, for example, the staple inserting unit has a lower surface having an inverted V-shaped guide groove, the staple receiving unit is accommodated in the saddle unit, and the staple receiving unit is configured to protrude upward from the top of the saddle unit when the staple inserting unit inserts a staple into the sheets.
In the above publication, the staple inserting unit aligns the folding lines of the sheets by the guide groove thereof upon the protrusion of the staple receiving unit.
According to the configuration of the above publication, the staple inserting unit inserts a staple into sheets that are not fixed to the saddle unit, while the staple receiving unit is protruding from the top of the saddle unit. Unfortunately, the staple inserting unit cannot correct the deviation due to the significant misalignment of the ridge line of the sheets caused upon the protrusion of the staple receiving unit, resulting in misalignment of the stitch with the folding lines of the sheets.
According to the configuration in the above publication, the sheets to be stapled define the same folding angles as those when accumulated on the saddle unit, and thus a space is generated between the staple receiving unit and the sheets. Unfortunately, the staple buckles if the space is significantly large. In addition, a longer penetration length of the staple is needed to staple the sheets defining the same folding angles as those when accumulated on the saddle unit, compared to that needed in a side stitching process. This causes a disadvantage in that a smaller number of sheets can be stitched than those in the side stitching process.